Sapping

a method of digging ditches and trenches (communication trenches) under the fire of the enemy in order to come closer to their fortifications; the method was employed in the 16th through 19th centuries.

There were masked (quiet) and flying saps. In excavating the trench of the masked sap, the work was carried on from the bottom of the initial ditch, and the workers did not come to the surface. In the flying sap, workers dug from the surface under the cover of a previously prepared protective embankment of barrels and bags of earth. In the second half of the 17th century the armies of several countries used specialists called sappers to dig trenches.

In a figurative sense, the Russian expression tikhoi sapoi (“by a quiet sap”) means to act surreptitiously or secretly; to infiltrate.